Recombinant bacterial lipocalin Blc and uses thereof

ABSTRACT

The present inventors have solved the crystal structure of an  Escherichia coli  bacterial lipocalin polypeptide, which depicts a monomeric protein. Previous crystal structures have been reported, but these appear to he inaccurate, as they predicted, e.g., a dimeric protein. The crystal structure of a bacterial lipocalin provided by the present invention leads to numerous uses. For example, the present invention provides for the design, construction and use of recombinant libraries of diversified bacterial lipocalins resulting from a bacterial lipocalin polypeptide “backbone”.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. Ser. No. 13/820,893, filed Aug. 22, 2013 (now pending), which is a National Stage application of PCT/EP2011/065423, filed Sep. 7, 2011, which claims the benefit of U.S. Provisional Application No. 61/380,521, filed Sep. 7, 2010.

The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-WEB and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Mar. 4, 2013, is named sequence.txt and is 10 KB.

BACKGROUND

Lipocalins represent a family of functionally diverse, small proteins comprising 160-180 residues that share high conservation at the tertiary structural level while having weak amino acid sequence homology (Flower et al., 2000; Skerra, 2000). Their fold is dominated by an eight-stranded antiparallel β-barrel with an α-helix attached to its side, whereby four structurally variable loops, which connect neighboring β-strands at the open end of the barrel, form the entrance to a ligand pocket. Further hallmarks are three structurally conserved regions (SCRs) (Flower, 1996), which assist the identification of new lipocalins at the primary structure level.

Lipocalins were initially described for eukaryotes and only more recently identified in Gram-negative bacteria (Flower, 1996; Bishop, 2000). The bacterial lipocalin (Blc) was first discovered in Escherichia coli (Bishop et al., 1995), but sequence analyses have indicated the existence of at least 20 other bacterial lipocalins, for example in Citrobacter freundii,Vibrio cholerae and many other Enterobacteriaceae.

Blc belongs to the class I outer membrane lipoproteins, carrying a type II signal peptide at the N-terminus, which directs export into the periplasm. After signal peptide processing, the protein becomes anchored into the inner leaflet of the outer membrane (Bishop et al., 1995) via a lipid-modified amino-terminal cysteine residue. The blc promoter is mainly induced at the onset of the stationary growth phase via the rpoS sigma factor, which generally directs gene expression for adaptation to starvation and high osmolarity or other conditions known to exert stress on the cell envelope. The blc gene is poorly transcribed, suggesting that the normal concentration of Blc in the outer membrane is low.

Other findings indicate an implication of Blc in bacterial host pathogenesis (Bishop, 2000). The blc genes of some Enterobacteriaceae are physically linked to the ampC gene, which encodes a serine β-lactamase on the chromosome but also appears to be genetically recombined into different plasmids. This co-localisation suggests that the blc gene may be involved in antibiotic resistance. Furthermore, bacterial lipocalins play a role in the host immune response as many components of the bacterial cell envelope provide so-called pathogen-associated molecular patterns for surveillance. One relevant component is the N-acyl-S-sn-1,2-diacylglycerylcysteine modification at the N-terminus of the bacterial lipoproteins, permitting macrophages and other immune cells to recognize Blc via CD14 and the Toll-like receptor 2.

The first crystal structure of an N-terminally extended version of a Blc, so-called Blc-X (Campanacci et al. 2004), revealed the β-barrel fold characteristic for the lipocalin family and was followed by a second crystal structure of Blc-X in complex with the fatty acid vaccenic acid (Campanacci et al., 2006). Both structures belong to the space group P2₁2₁2₁ with isomorphous unit cell parameters and an overall r.m.s. deviation of 0.1 Å for 167 Cα atoms.

The crystal structures of Campanacci et al. indicated that Blc was a dimeric protein. This prediction was based on the identification of a tight pairwise contact of several side chains and a buried surface of 786 Å² and 825 Å², respectively, of the two distinct Blc-X molecules A and B within the asymmetric unit, and also on static light scattering measurements in solution. Notably, the fatty acid ligand was bound in the cavity of just one molecule of the dimer and involved in only a few additional contacts to the other molecule, which was explained by the asymmetric interaction of the two Blc-X monomers. Yet, binding of vaccenic acid did not lead to detectable conformational changes within the Blc-X dimer (Campanacci et al., 2006).

The present inventors, however, studied the biochemistry and structure of a recombinant Blc without N-terminal extension overproduced with a different E. coli expression vector, and found striking evidence that Blc behaves as a stable monomer in solution. The previously described dimerization is, therefore, likely the result of a cloning artifact. This surprising finding is one of the bases of the present invention.

SUMMARY OF THE INVENTION

The present invention can be summarized by the following items

-   1. A method of preparing a plurality of nucleic acid molecules,     comprising the step of (i) synthesizing or (ii) recombinantly     producing a plurality of nucleic acid molecules, wherein said     molecules differ from each other in at least one nucleotide at a     position within at least one loop region of the nucleic acid     encoding a Blc polypeptide. -   2. A method according to item I, comprising the step of synthesizing     or recombinantly producing at least six different nucleic acid     molecules, -   3. A method according to any one of the preceding items, wherein     said molecules differ from each other in at least one nucleotide at     a position within at least two loop regions of the nucleic acid     encoding said Blc polypeptide. -   4. A method according to any one of the preceding items, wherein     said molecules differ from each other in at least one nucleotide at     a position within at least three loop regions of the nucleic acid     encoding said Blc polypeptide. -   5. A method according to any one of the preceding items, wherein     said molecules differ from each other in at least one nucleotide at     a position within at least four loop regions of the nucleic acid     encoding said Blc polypeptide. -   6. A method according to any one of the preceding items, wherein     said molecules differ from each other in at least one nucleotide at     a position within the nucleotides encoding amino acid residue     positions 88-96 of said Blc polypeptide, and wherein said molecules     encode different polypeptides from each other. -   7. A method according to any one of the preceding items, wherein     said molecules differ from each other in at least one nucleotide at     a position within the nucleotides encoding amino acid residue     positions 27-43 of said Blc polypeptide, and wherein said molecules     encode different polypeptides from each other. -   8. A method according to any one of the preceding items, wherein     said molecules differ from each other in at least one nucleotide at     a position within the nucleotides encoding amino acid residue     positions 58-73 of said Blc polypeptide, and wherein said molecules     encode different polypeptides from each other. -   9. A method according to any one of any one of the preceding items,     wherein said molecules differ from each other in at least one     nucleotide at a position within the nucleotides encoding amino acid     residue positions 113-121 of said Blc polypeptide and wherein said     molecules encode different polypeptides from each other. -   10. A method of preparing a plurality of polypeptide molecules,     comprising the steps of expressing the plurality of nucleic acid     molecules according to any one of the preceding items. -   11. A composition comprising a plurality of nucleic acid molecules,     wherein said molecules differ from each other in at least one     nucleic acid nucleotide at a position within at least one loop     region of the nucleic acid encoding a Blc polypeptide. -   12. A composition according to item 11, wherein said plurality     comprises at least six different nucleic acid molecules. -   13. A composition according to item 11 or 12, wherein said nucleic     acid molecules differ from each other in at least one nucleotide at     a position within at least two loop regions of the nucleic acid     encoding said Blc polypeptide. -   14. A composition according to any one of items 11 to 13, wherein     said molecules differ from each other in at least one position     within at least three loop regions of the nucleic acid encoding said     Blc polypeptide. -   15. A composition according to any one of items 11 to 14, wherein     said molecules differ from each other in at least one nucleotide at     a position within at least four loop regions of the nucleic acid     encoding said Blc polypeptide. -   16. A composition according to any one of items 11 to 15, wherein     said molecules differ from each other in at least one nucleotide at     a position within the nucleotides encoding amino acid residue     positions 88-96 of said Blc polypeptide, and wherein said molecules     encode different polypeptides from each other. -   17. A composition according to any one of items 11 to 16, wherein     said molecules differ from each other in at least one nucleotide at     a position within the nucleotides encoding amino acid residue     positions 27-43 of said Blc polypeptide. -   18. A method according to any one of items 11 to 17, wherein said     molecules differ from each other in at least one nucleotide at a     position within the nucleotides encoding amino acid residue     positions 58-73 of said Blc polypeptide -   19. A method according to any one of items 11 to 18, wherein said     molecules differ from each other in at least one nucleotide at a     position within the nucleotides encoding amino acid residue     positions 113-121 of said Blc polypeptide. -   20. A composition comprising a plurality of polypeptide molecules,     wherein said polypeptide molecules differ from each other in at     least one amino acid at a position within at least one loop region     of a Blc polypeptide. -   21. A composition according to item 20, wherein said plurality     comprises at least six different polypeptide molecules. -   22. A composition according to item 20 or 21, wherein said     polypeptide molecules differ from each other in at least one amino     acid at a position within at least two loop regions of said Blc     polypeptide. -   23. A composition according to any one of items 20 to 22, wherein     said polypeptide molecules differ from each other in at least one     amino acid at a position within at least three loop regions of said     Blc polypeptide. -   24. A composition according to any one of items 20 to 23, wherein     said polypeptide molecules differ from each other in at least one     amino acid at a position within at least four loop regions of said     Blc polypeptide -   25. A composition according to any one of items 20 to 24, wherein     said molecules differ from each other in at least one amino acid     position within amino acid residue positions 88-96 of said Blc     polypeptide. -   26. A composition according to any one of items 20 to 25, wherein     said molecules differ from each other in at least one amino acid     position within amino acid residue positions 27-43 of said Blc     polypeptide. -   27. A method according to any one of items 20 to 26, wherein said     molecules differ from each other in at least one amino acid position     within amino acid residue positions 58-73 of said Blc polypeptide -   28. A method according to any one of items 20 to 27, wherein said     molecules differ from each other in at least one amino acid position     within amino acid residue positions 113-121 of said Blc polypeptide. -   29. A method of isolating a polypeptide of interest, comprising the     steps of (i) allowing the composition of any one of items 20 to 28     to be in contact with a target of interest, wherein said polypeptide     of interest is comprised within said composition and specifically     binds to the target; and (ii) isolating the polypeptide or a     polypeptide-target complex resulting from said specific binding -   30. A method of isolating a nucleic acid molecule of interest,     comprising the steps of (i) allowing the composition of any one of     items 20 to 28 to be in contact with a target of interest, where     said polypeptide is comprised within said composition and     specifically binds to the target; (ii) isolating the polypeptide or     a polypeptide-target complex resulting from said specific     binding; (iii) isolating the nucleic acid sequence encoding said     polypeptide; and (iv) determining the nucleic acid encoding said     polypeptide. -   31. A crystal structure of a monomeric Blc polypeptide. -   32. A crystal structure according to item 31, wherein said structure     is depicted in FIG. 3. -   33. A method according to any one of items 1-10, 18-17, and 27-30     wherein said molecules are selected from a group consisting of at     least 50%, at least 60%, at least 75%, at least 90%, at least 95%,     or at least 99 sequence identity with SEQ-ID NO. 1, -   34. A composition according to any one of items 11-17 and 20-26     wherein said molecules are selected from a group consisting of at     least 50%, at least 60%, at least 75%, at least 90%, at least 95%,     or at least 99% sequence identity with SEQ-ID NO. 1. -   35. An isolated crystalline form of a monomeric Blc polypeptide -   36. An isolated crystalline form of item 35, wherein said     crystalline form has a I4122 space group and unit cell dimensions     a=b=88.9, c=78.4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Comparison of Blc (dark grey) crystallized in space group I4₁22, with Blc-X (light grey) crystallized in space group P2₁2₁2₁ (PDB entry 2ACO, depicting the two monomers in the asymmetric unit) after superposition of the 149 Cα positions resolved for all structures. The previously published structure of Blc-X reveals a long N-terminal extension including an additional β-strand and a hairpin loop (black). N-Blc denotes the first residue visible in the crystal structure for Blc, N-BlcX denotes the first residue visible in the crystal structure for Blc-X, C labels the C-termini of all three structures shown.

FIG. 2: Analysis of the dimer interface in the previously described Blc-X crystal structure with two monomers in the asymmetric unit (PDB entry 2ACO). The ligand vaccenic acid (labeled VCA, grey spheres) occupies only one ligand pocket. The N-terminal peptide extension of Blc-X, which includes the hairpin loop and the additional β-strand outside the β-barrel, is colored black for both monomers. The four mutated residues within the hairpin loop of Blc-X (Lys-Ala-Gly-Ser), whose sequence positions correspond to the four N-terminal amino acids of mature wild-type Blc, are depicted with side chains and labeled for the rightmost monomer B. This segment of molecule B forms direct contacts with three side chains (Tyr94, Lys105, Tyr113) displayed on the β-barrel surface of molecule A (left). Next to this interaction, the E/F-loops of both molecules form another tight, almost symmetrical contact. N- and C-termini of the polypeptide chains are labeled.

FIG. 3: Crystalline structure of the invention elucidated by X-ray diffraction. Amino acid positions 27-43, 58-73, 88-96, 113-121 represent the loop regions of the mature E. coli Blc.

FIG. 4: SEQ ID NO. 1: Full length recombinant Blc as encoded on pBlc2, including the OmpA signal peptide (amino acid residues 1-21) and the C-terminal Strep-tag. SEQ ID NO. 2: Amino acid sequence of full length recombinant Blc as encoded by SEQ ID NO. 1.

DETAILED DESCRIPTION

Unless otherwise specified, “a” or “an” means “one or more.” The disclosure illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including,” containing”, etc. shall be read expansively and without limitation. Also, the term “comprising” when used herein can be replaced by the term “consisting of”. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible. Thus, it should be understood that although the present disclosure has been specifically disclosed by exemplary embodiments and optional features, modification and variation of the disclosures embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this disclosure. The compositions, methods, procedures, treatments, molecules and specific compounds described herein are presently representative of preferred embodiments are exemplary and are not intended as limitations on the scope of the invention. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention are defined by the scope of the claims. The listing or discussion of a previously published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

The present inventors have solved the crystal structure of a particular Escherichia coli bacterial lipocalin (Blc), as depicted in dark grey in FIG. 1, which allows for several novel application as further described herein. As used herein, “Blc” is defined as any bacterial lipocalin, including but not limited to the bacterial lipocalin having SwissProt Entry P0A901. Blc polypeptides comprise a class of polypeptides that belongs to the class I outer is membrane lipoproteins, carrying a type II signal peptide at the N-terminus which allows export into the periplasm. The amino acid sequence of the Blc polypeptide is shown in SEQ ID NO.3, with amino acids 1-18 representing the type II signal peptide and amino acids 19-177 representing the mature bacterial lipocalin protein. After signal peptide processing, the protein becomes anchored into the inner leaflet of the outer membrane (Bishop et al., 1995) via a lipid-modified amino-terminal cysteine residue. SEQ ID NO. 4 shows the amino acid sequence of the mature bacterial lipocalin protein.

Accordingly, the invention relates to a method of preparing a plurality of nucleic acid molecules, comprising the step of (i) synthesizing or (ii) recombinantly producing a plurality of nucleic acid molecules, wherein said molecules differ from each other in at least one nucleotide at a position within at least one loop region of the nucleic acid encoding a Blc polypeptide. The nucleic acids of the plurality of nucleic acids encode one or more bacterial lipocalin (Blc) muteins.

Also, the invention relates to a composition comprising a plurality of nucleic acid molecules, wherein said molecules differ from each other in at least one nucleic acid nucleotide at a position within at least one loop region of the nucleic acid encoding a Blc polypeptide. Similarly, the invention relates to a plurality of polypeptide molecules, wherein said polypeptide molecules differ from each other in at least one amino acid at a position within at least one loop region of a Blc polypeptide. These polypeptide molecules are in the sense of the invention bacterial lipocalin muteins.

Additional embodiments relate to novel crystals of monomer Blc.

The amino acid sequence shown in SEQ ID NO 1 (or 2) comprises at amino acid position 22-180 the mature bacterial lipocalin protein, while amino acids 1-21 represent the type I OmpA signal peptide and amino acids 181-189 represent the Strep-tag II

For the purpose of the present invention whenever reference is made to bacterial lipocalin, the mature bacterial lipocalin shown, for example, in SEQ ID NO. 1 at amino acid positions 22-180), SEQ ID NO. 2 at amino acid positions 22-180, SEQ ID NO. 3 at amino acid positions 19-177 or SEQ ID NO. 4 at amino acid positions 1-159 is meant. Thus, when locating, for example, the amino acid residue positions 27-43, 58-73, 88-96 or 113-121 of the loops of the bacterial lipocalin of the present invention should be located, the starting point for counting is in SEQ ID NO. 1 and SEQ ID NO. 2 amino acid position 22 (i.e., position 22 of SEQ ID NO. 1 or 2 is position “I” for that purpose), in SEQ ID NO. 3 amino acid position 19 (i.e., position 19 of SEQ ID NO. 3 is position “1” for that purpose) and in SEQ ID NO. 4 amino acid position 1 (i e., position 1 of SEQ ID NO. 4 is position “1” for that purpose).

In one aspect, the present invention relates to a library of functional polypeptides that is based on the Blc lipocalin; accordingly, the invention includes polypeptides that have at least 50%, preferably at least 75%, more preferably at least 90%, and most preferably at least 99 sequence identity with SEQ. ID NO. 1 or 3. It is preferred that the degree of identity is determined over the full length of the sequence being compared. It is also preferred that polypeptides that have the degree identity as described herein with SEQ ID NO. 1 or 3 have one or more amino acid replacements, in particular in the loops (1, 2, 3 and/or 4) in comparison to the wildtype bacterial lipocalin. The use of various lipocalins as a “progenitor” to develop a library of lipocalin polypeptides that have diversified amino acids compared to the progenitor has been described (see, e.g., Beste et al, (1999)). Indeed, libraries of human lipocalin-derived proteins, called Anticalins, have led to the isolation of several therapeutically relevant drug candidates (see, e.g. WO05/019256, WO06/056464).

Prior to the proper elucidation of the Blc polypeptide by the present inventors, however, a rational design of a diverse library of Blc-derived Anticalins was not possible. For instance, based on the previously proposed crystal structure of Blc, Campanacci et al predicted that Blc would former homodimers, which would not have motivated the skilled worker to design a library based on Blc, given, e.g., the inherent complexities of utilizing a dimeric lipocalin over the known monomeric lipocalins whose structures already have been solved.

According to the previously reported structures, as shown in FIG. 2, the skilled worker would have been motivated, for example, against diversifying the E/F and G/H loop regions in a library of diversified Blc polypeptides, as the skilled worker would have expected these regions to form part of or be close to, the interface of the proposed Blc dimer.

Accordingly, an accurate crystal structure of a bacterial lipocalin, which is one aspect of the present invention, provides numerous applications, which also are provided by the present invention. For example, the present invention provides for the design, construction and use of recombinant libraries of diversified bacterial lipocalins (including nucleic acid molecules encoding the same) modelled after a bacterial lipocalin polypeptide “backbone” such as Blc. Techniques for recombinantly producing proteins are known to those skilled in the art (e.g. Skerra, A. (2001)).

Thus, in general, the present invention contemplates a monomeric Blc protein having the amino acid sequence shown in SEQ ID NO. 3. Also, it is generally preferred that the bacterial lipocalin muteins of the present invention are monomeric.

In one embodiment, the present invention provides a method of preparing a plurality of nucleic acid molecules that are based on a Blc polypeptide and that differ from each other. As used herein, a “plurality” is defined as two or more. In a preferred embodiment, the plurality includes at least six nucleic molecules and may include at least 50, 10^2, 10^3, 10^4, 10^5, 10^6, 10^7, 10^8, 10^9, 10^10, or at least 10^11 nucleic acid molecules.

The present invention also contemplates a method of preparing a plurality of polypeptide molecules that are based on a Blc polypeptide and that differ from each other. In this sense, the present invention includes a method of expressing the plurality of nucleic acids molecules that are based on a Blc polypeptide and contemplated herein.

Additionally, the present invention provides compositions that include a plurality of nucleic acid molecules that are based on a Blc polypeptide and that differ from each other. Within the plurality of nucleic acid molecules contemplated by the present invention (as well as the methods described herein), the molecules may differ from each other in any region. The plurality of nucleic acid molecules may, for example, differ in a loop region of a Blc polypeptide. In this sense, the plurality of nucleic acid molecules may, for instance, differ in at least one nucleotide at a position within one, two, three and/or four loop regions of the nucleic acid encoding a Blc polypeptide. These positions may, but are not required to, include at least one nucleotide at a position within the nucleotides encoding amino acid residue positions 27-43, 58-73, 88-96, 113-121 of a Blc polypeptide. Preferably, said molecules, substantially all of said molecules or the majority of said molecules encodes different polypeptides from each other. As used herein, a “loop region” of a Blc polypeptide is defined as a peptide segment which joins two adjacent β-strands that form part of the β-barrel in the lipocalin structure, which may include adjoining parts of the β-strands themselves. Examples for such loop regions are the segments comprising amino acid residue positions 27-43, 58-73, 88-96 or 113-121 of a Blc polypeptide.

The amino acid sequence for wild type Blc, including its signal sequence, (SEQ ID NO:3) is:

  1 mrllplvaaa taaflvvaCS SPTPPRGVTV VNNFDAKRYL GTWYEIARFD HRFERGLEKV  61 TATYSLRDDG GLNVINKGYN PDRGMWQQSE GKAYFTGAPT RAALKVSFFG PFYGGYNVIA 121 LDREYRHALV CGPDRDYLWI LSRTPTISDE VKQEMLAVAT REGFDVSKFI WVQQPGS

The lower-case letters represent the signal peptide, while the underlined letters represent the Blc loops that can be randomized according to the present invention.

Additionally, the present invention provides compositions that include a plurality of polypeptides. In this sense, the present invention provides, for example, compositions that include a plurality of amino acid sequences that are based on a Blc polypeptide and that differ from each other. Within the plurality of polypeptides and, hence, amino acid sequences contemplated by the present invention (as well as the methods described herein), the molecules may differ from each other in any region. The plurality of polypeptides may, for example, differ in a loop region of a Blc polypeptide. In this sense, the plurality of polypeptides may, for instance, differ in at least one amino acid at a position within one, two, three and/or four loop regions of a Blc polypeptide. These positions may, but are not required to, include at least one position within the amino acid residue positions 27-43, 58-73, 88-96, 113-121 of a Blc polypeptide. Preferably, said molecules, substantially all of said molecules or the majority of said molecules encodes different polypeptides from each other.

The present invention also contemplates the preparation of a library of Blc variants that contains a plurality of polypeptide molecules, where polypeptide molecules may contain one or more different amino acids (vis-a-vis a mature Blc polypeptide) within any or all of the 4 loop region positions, as delineated in FIG. 3.

The present invention also provides a method of isolating a polypeptide of interest, as described in Kim et al. (2009) and Schönfeld et al. (2009). In one aspect, this includes a step of allowing a composition of the present invention to be in contact with a target of interest. To this end, a polypeptide of interest is comprised within the composition and specifically binds to the target. As used herein, a “target” is defined as any molecule to which a polypeptide of the invention is capable of specifically binding, including all types of proteinaceous and non-proteinacious molecules such as haptens or other small molecules. As used herein, a polypeptide of the invention “specifically binds” a target if it is able to discriminate between that target and one or more reference targets, since binding specificity is not an absolute, but a relative property, “Specific binding” can be determined, for example, in accordance with Western blots, ELISA-, RIA-, ECL-, IRMA-tests, FACS, IHC and peptide scans. The polypeptide of the invention can bind to the target with an affinity in the micromolar or, in more preferred embodiments, in the nanomolar range. Binding constants of less than 100 μM, 50 μM, 500 nM, 250 nM, 100 nM and 50 nM are also envisioned for the current invention.

According to this method, a polypeptide that specifically binds to a target of interest is preferably isolated, which can be accomplished according to conventional techniques, such as those defined in Kim et al. (2009) and Schönfeld et al. (2009), which typically involve the selection of a polypeptide of interest (phenotype) that is linked to its underlying genetic code (genotype). For instance, the polypeptide of interest may be expressed and then selected as part of any conventional selection technology, for example, a technology that involves a replicable genetic package, such as a bacteriophage. These technologies include, for example, display technologies such as phage display and cell surface display. In vitro selection technologies, such as ribosome display, also may be used. See, e.g., Kawasaki, U.S. Pat. Nos. 5,643,768 and 5,658,754.

The present invention also provides a crystal structure of a monomeric Blc polypeptide. In a particular embodiment, the present invention includes the crystal structure is depicted in FIGS. 1 and 3. The present invention also includes a Blc crystal being characterized by the data shown in Table 1.

A preferred embodiment of the method for preparing a plurality of nucleic acid molecules includes the generation of a mutein of a bacterial lipocalin protein, said mutein having detectable affinity to a given target, comprising the step of (a) subjecting the bacterial lipocalin to mutagenesis at one or more of the sequence positions which correspond to the sequence positions 27 to 43 (loop 1), 58-73 (loop 2), 88-96 (loop 3) and/or 113 to 121 (loop 4) of SEQ ID NO. 3, resulting in one or more mutein (s) of the bacterial lipocalin protein.

In a preferred embodiment, said method further comprises step (b) enriching at least one resulting mutein having binding affinity for a given target from the one or more muteins by selection and/or isolating said at least one mutein.

Preferably, the mutagenesis in step (a) of the method for generating a mutein of a bacterial lipocalin results in a plurality of muteins of the protein.

It is also a preferred embodiment of the present invention that further amino acids of the bacterial lipocalin protein (apart from those comprised by the four loops) are subjected to mutagenesis in the above method.

The composition of the present invention comprising a plurality of polypeptide molecules preferably includes a Blc mutein in which one or more amino acids within one, two, three, or all four loops are changed in comparison to the wildtype (or reference) Blc of the present invention. Said one or more amino acids include 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more amino acids that can be changed in loop 1, loop 2, loop 3 and loop 4 However, it is also envisaged that within loop 1 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or 17; within loop 2 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16; within loop 3 1, 2, 3, 4, 5, 6, 7, 8, or 9; and/or within loop 4 1, 2, 3, 4, 5, 6, 7, 8, or 9 amino acids are replaced. “Replacement” means that an amino acid different from that present at the corresponding position in the wildtype (or reference) bacterial lipocalin is present in a bacterial lipocalin of the present invention,

The present invention also envisages a bacterial lipocalin other than Blc in which one or more amino acids within one, two, three, or all four loops are changed in comparison to the wildtype (or reference) bacterial lipocalin of the present invention. Said one or more amino acids include 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more amino acids that can be changed in loop 1, loop 2, loop 3 and loop 4. However, it is also envisaged that within loop 1 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or 17; within loop 2 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16; within loop 3 1, 2, 3, 4, 5, 6, 7, 8, or 9; and/or within loop 4 1, 2, 3, 4, 5, 6, 7, 8, or 9 amino acids are replaced.

The loops of a bacterial lipocalin other than Blc can be readily identified by the skilled person as described herein. Briefly, the skilled person can align the Blc of the present invention and an amino acid sequence of interest that is assumed to be a bacterial lipocalin so as to determine (i) the degree of identity and/or (ii) the position of the loops that correspond to those of the loops of Blc.

When used herein, a “mutein,” a “mutated” entity (whether protein or nucleic acid) or “mutant” refers to the exchange, deletion, or insertion of one or more nucleotides or amino acids, respectively, within the bacterial lipocalin protein (Blc) of the present invention compared to the naturally occurring (wild-type) nucleic acid or protein “reference” scaffold of Blc, for example, shown in SEQ ID NO. 3.

Accordingly, a mutein of the invention may include the wild type (natural) amino acid sequence of the “parental” protein scaffold (bacterial lipocalin (Blc)) outside the mutated one or more amino acid sequence positions within one, two, three or four loop(s); alternatively, a bacterial lipocalin mutein may also contain amino acid mutations outside the sequence positions subjected to mutagenesis that do not interfere with the binding activity and the folding of the mutein. Such mutations can be accomplished on a DNA level using established standard methods (Sambrook, J. et al. (2001) Molecular Cloning: A Laboratory Manual, 3^(rd) Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). Possible alterations of the amino acid sequence are insertions or deletions as well as amino acid substitutions.

Such substitutions may be conservative, i.e. an amino acid residue is replaced with a chemically similar amino acid residue. Examples of conservative substitutions are the replacements among the members of the following groups: 1) alanine, serine, and threonine; 2) aspartic acid and glutamic acid; 3) asparagine and glutamine; 4) arginine and lysine; 5) isoleucine, leucine, methionine, and valine; and 6) phenylalanine, tyrosine, and tryptophan. One the other hand, it is also possible to introduce non-conservative alterations in the amino acid sequence. In addition, instead of replacing single amino acid residues, it is also possible to either insert or delete one or more continuous amino acids of the primary structure of a parental protein scaffold, where these deletions or insertion result in a stable folded/functional mutein, which can be readily tested by the skilled worker.

The skilled worker will appreciate methods useful to prepare protein muteins contemplated by the present invention but whose protein or nucleic acid sequences are not explicitly disclosed herein. As an overview, such modifications of the amino acid sequence include, e.g., directed mutagenesis of single amino acid positions in order to simplify sub-cloning of a mutated lipocalin gene or its parts by incorporating cleavage sites for certain restriction enzymes. In addition, these mutations can also be incorporated to further improve the affinity of a lipocalin mutein for a given target. Furthermore, mutations can be introduced to modulate certain characteristics of the mutein such as to improve folding stability, serum stability, protein resistance or water solubility or to reduce aggregation tendency, if necessary. For example, naturally occurring cysteine residues may be mutated to other amino acids to prevent disulphide bridge formation.

Accordingly, the invention also includes functional variants of muteins disclosed herein, which have a threshold sequence identity or sequence homology to a reference protein. By “identity” or “sequence identity” is meant a property of sequences that measures their similarity or relationship. The term “sequence identity” or “identity” as used in the present invention means the percentage of pair-wise identical residues—following (homologous) alignment of a sequence of a polypeptide of the invention with a sequence in question—with respect to the number of residues in the longer of these two sequences. Percent identity is determined by dividing the number of identical residues by the total number of residues and multiplying the product by 100. The term “homology” is used herein in its usual meaning and includes identical amino acids as well as amino acids which are regarded to be conservative substitutions (for example, exchange of a glutamate residue by an aspartate residue) at equivalent positions in the linear amino acid sequence of two proteins. Most preferred, the amino acid sequence shown in SEQ ID NO. 3 is preferred as a “reference sequence”. SEQ ID NO. 3 shows the mature bacterial lipocalin protein (Blc). The term “reference sequence” and “wild type sequence” (of Blc) is used interchangeably herein.

As mentioned herein, the present invention relates to polypeptides that share a certain degree of identity with the bacterial lipocalin protein (Blc) of the present invention. Such polypeptides comprise preferably one or more amino acid replacements in comparison to the wildtype Blc amino acid sequence.

The percentage of sequence homology or sequence identity can, for example, be determined herein using the program BLASTP, version blastp 2.2.5 (Nov. 16, 2002; cf. Altschul, S. F. et al. (1997) Nucl. Acids Res. 25, 3389-3402). In this embodiment the percentage of homology is based on the alignment of the entire polypeptide sequences (matrix: BLOSUM 62; gap costs: 11.1; cutoff value set to 10⁻¹) including the propeptide sequences, preferably using the wild type protein scaffold as reference in a pairwise comparison. It is calculated as the percentage of numbers of “positives” (homologous amino acids) indicated as result in the BLASTP program output divided by the total number of amino acids selected by the program for the alignment.

It is also possible to deliberately mutate other amino acid sequence positions to cysteine in order to introduce new reactive groups, for example, for the conjugation to other compounds, such as polyethylene glycol (PEG), hydroxyethyl starch (HES), biotin, peptides or proteins, or for the formation of non-naturally occurring disulphide linkages. With respect to a mutein of human Lipocalin 2, exemplary possibilities of such a mutation to introduce a cysteine residue into the amino acid sequence of a bacterial lipocalin mutein.

The term “position” when used in accordance with the invention means the position of either an amino acid within an amino acid sequence depicted herein or the position of a nucleotide within a nucleic acid sequence depicted herein. The term “corresponding” as used herein also includes that a position is not only determined by the number of the preceding nucleotides/amino acids. Accordingly, the position of a given amino acid in accordance with the invention which may be substituted may vary due to deletion or addition of amino acids elsewhere in a (mutant or wild-type) lipocalin. Similarly, the position of a given nucleotide in accordance with the present invention which may be substituted may vary due to deletions or additional nucleotides elsewhere in a mutein or wild type bacterial lipocalin 5′-untranslated region (UTR) including the promoter and/or any other regulatory sequences or gene (including exons and introns).

Thus, under a “corresponding position” in accordance with the invention it is preferably to be understood that nucleotides/amino acids may differ in the indicated number but may still have similar neighboring nucleotides/amino acids. Said nucleotides/amino acids which may be exchanged, deleted or added are also comprised by the term “corresponding position”. When used herein “at a position corresponding to a position” a position in a “query” amino acid (or nucleotide) sequence is meant that corresponds to a position in a “subject” amino acid (or nucleotide) sequence.

Specifically, in order to determine whether a nucleotide residue or amino acid residue of the amino acid sequence of a bacterial lipocalin different from a bacterial lipocalin mutein of the invention corresponds to a certain position in the nucleotide sequence or the amino acid sequence of a bacterial lipocalin mutein as described, in particular any of SEQ ID NOs. 1-4, a skilled artisan can use means and methods well-known in the art, e.g., alignments, either manually or by using computer programs such as BLAST 2.0 (Altschul et al. (1990), J. Mol. Biol. 215:403-10), which stands for Basic Local Alignment Search Tool, or ClustalW (Thompson et al (1994), Nucleic Acids Res. 22(22):4673-80) or any other suitable program which is suitable to generate sequence alignments. Accordingly, a bacterial lipocalin mutein of any of SEQ ID NOs 1-4 can serve as “subject sequence”, while the amino acid sequence of a bacterial lipocalin different from Blc as described herein serves as “query sequence”.

Given the above, a skilled artisan is thus readily in a position to determine which amino acid position mutated in Blc as described herein corresponds to an amino acid of a bacterial lipocalin scaffold other than Blc. Specifically, a skilled artisan can align the amino acid sequence of a mutein as described herein, in particular a Blc mutein of the invention with the amino acid sequence of a different bacterial lipocalin to determine which amino acid(s) of said mutein correspond(s) to the respective amino acid(s) of the amino acid sequence of said different lipocalin.

When used herein “a bacterial lipocalin other than Blc” or “a bacterial lipocalin different from Blc” in which one or more amino acid replacements, in particular at one or more position in one, two, three or all four loops, can be made in accordance with the teaching of the present invention encompasses any other bacterial lipocalin known in the art or which can be identified by using Blc as reference sequence, for example, in a BLAST search or using a nucleic acid molecule encoding Blc as probe in, for example, a hybridization experiment.

Preferred bacterial lipocalin scaffolds other than Blc in which one or more amino acid replacements, in particular at one or more position in one, two, three or all four loops, can be made in accordance with the teaching of the present invention can, for example, be retrieved from the bacteria shown in the following Table.

A bacterial lipocalin scaffold other than Blc can be retrieved via the accession number from the bacteria listed in the following Table as follows: A TBLASTN search (Altschul et al., cited herein) using the Blc protein sequence is to be performed on both the microbial genome databases and the non-redundant database at the NCBI website (http://www.ncbi.nlm.nih.gov). The genome sequence 500 bp upstream and downstream of each hit should be retrieved. The six resulting translations are evaluated to identify both the full-length lipocalin sequence and the presence of the 16s ribosomal RNA binding site.

Accession Bacterial species Number Agrobacterium NC_003063 tumefaciens Caulobacter crescentus NC_002696 Mesorhizobium loti NC_002678 Rhodobacter AAAE01000158 sphaeroides Rhodospirillum rubrum AAAG0200000 Silicibacter NZ_AAFG01000010 Acidovorax (partial) AB044565 Bordetella NC_002927 Bordetella parapertussis NC_002928 Burkholderia cepacia AAEH01000003 Burkholderia fungorum NZ_AAAJ03000001 Chromobacterium NC_005085 violaceum Dechloromonas AADF01000001 aromatica Methylobacillus AADX01000001 flagellatus Ralstonia eutropha AADY01000001 Rubrivivax gelatinosus AAEM01000005 Thiobacillus AAFH01000001 denitrificans Citrobacter braakii AF492447 (partial) Citrobacter freundii U21727 Citrobacter murliniae AJ607409 Enterobacter AJ487975 nimipressuralis Enwinia carotovora BX950851 Escherichia coli P39281 Francisella tularensis AY774926 Idiomarina loihiensis NC_006512 Klebsiella oxytoca Y17716 Pseudomonas AABQ0700000 Pseudomonas AAAT0300000 Pseudomonas putida NC_002947 Pseudomonas syingae AABP02000002 Salmonella typhi NC_006511 Salmonella typhimurium AE008903 Shewanella oneidensis AE015615 Shigella flexneri NC_004741 Vibrio cholerae NC_002506 Rubrivivax gelatinosus AAEM0100000 Thiobacillus denitrificans AAFH01000001 Acinetobacter CR543861 Azotobacter vinelandii NZ_AAAU02000004 Citrobacter braakii AF492447 (partial) Citrobacter freundii U21727 Citrobacter murliniae AJ607409 Enterobacter AJ487975 Erwinia carotovora BX950851 Escherichia coli P39281 Francisella tularensis AY774926 Idiomarina loihiensis NC_006512 Klebsiella oxytoca Y17716 Pseudomonas aeruginosa AABQ07000004 Pseudomonas fluorescens AAAT03000001 Pseudomonas putida NC_002947 Acinetobacter CR543861 Azotobacter vinelandii NZ_AAAU02000004 Vibrio parahaemolyticus NC_004605 Vibrio vulnificus NC_004460 Xanthomonas axonopodis NC_003919 (1) Xanthomonas campestris NC_003902 (1) Bdellovibrio NC_005363 Desulfotalea psychrophila NC_006138 Geobacter sulfurreducens NC_002939 Campylobacter jejuni AL139078 Campylobacter lari NZ_AAFK01000002 Chlorobium lepidum NC_002932 Bacteroides fragilis NC_006347 Bacteroides NC_004663 Cytophaga hutchinsonii AABD03000002 Parachlamydia NC_005861 Gloeobacter violaceus NC_005125 Corynebacterium efficiens NC_004369 Corynebacterium NC_003450 glutamicum Nocardia farcinica NC_006361 Bacillus subtilis P54945

The present invention also relates to a nucleic acid coding for the one or more mutein (s) of the bacterial lipocalin protein, which nucleic acid results from mutagenesis. Preferably, said nucleic acid is operably fused at the 3′end a gene coding for the coat protein pIII of a filamentous bacteriophage of the M13-family or coding for a fragment of this coat protein, in order to select at least one mutein for the binding of the given target.

in another aspect, the present invention relates a bacterial lipocalin mutein as described herein which is conjugated to a label selected the group consisting of an organic molecule, an enzyme label, radioactive label, fluorescent label, chromogenic label, luminescent label, a hapten, digoxigenin, biotin, metal complexes, metals, and colloidal gold.

In yet another aspect, the present invention relates to a fusion protein comprising a bacetrial lipocalin mutein or the present invention, wherein an enzyme, a protein or a protein domain, a peptide, a signal sequence and/or an affinity tag is operably fused to the amino terminus or the carboxy terminus of said bacterial lipocalin mutein

Also, the present invention relates to a nucleic acid molecule encoding said fusion protein.

Moreover, the present invention relates to a pharmaceutical composition comprising a bacterial lipocalin mutein as described herein or a fusion protein as described herein and a pharmaceutically acceptable carrier or excipient

Furthermore, the present invention relates to a method for producing a bacterial lipocalin mutein or a fusion protein thereof, wherein the mutein or the fusion protein thereof is produced starting from the nucleic acid encoding the mutein by means of genetic engineering methods in a bacterial or eukaryotic host organism and is isolated from this host organism or its culture.

Finally, the present invention contemplates a use of a mutein of bacterial lipocalin or a fusion protein thereof for the detection of a given target, comprising the steps of contacting the mutein with a sample suspected of containing the given target under suitable conditions, thereby allowing formation of a complex between the mutein and the given target, and determining the complexed mutein by a suitable signal. The given target may be a protein or protein domain, a peptide, a nucleic acid molecule, an organic molecule or a metal complex and the detection is preferably carried out for validation of the protein as pharmacological drug target.

All references cited herein are hereby incorporated in their entirety.

The present invention is further illustrated by, though in no way limited to, the following examples.

EXAMPLES Example 1 Recombinant Expression and X-Ray Structure Determination of Blc

Blc was secreted as a soluble protein into the periplasm of E. coli after its original type II signal peptide had been exchanged by the type I signal peptide of OmpA (Ghrayeb et al., 1984), which has proven useful for recombinant protein export. In addition, the Cys residue at position 1 of the mature polypeptide, which otherwise carries the lipid anchor of the natural lipoprotein, was replaced by Ala and the unpaired internal thiol residue Cys 113 was substituted by Ser. Purification from the bacterial periplasmic extract was achieved via streptavidin affinity chromatography employing the Strep-tag II (Schmidt & Skerra, 2007), which had been appended to the C-terminus. Blc was finally obtained as a homogeneous protein by preparative gel filtration.

During this purification step we noted that our recombinant Blc elutes as a fully monomeric protein. This was confirmed by analytical size exclusion chromatography (SEC; data not shown), revealing an apparent size of 13.0 kDa, which was even smaller than the calculated mass of 19.1 kDa for the mature protein and clearly indicating the absence of a dimer. Consequently, its oligomerization behaviour was further investigated by means of analytical ultracentrifugation (AUC), resulting in a monomeric molecular mass of 18.7±0.4 kDa. This demonstrated that our recombinant Blc forms a stable monomer in solution, at least up to a concentration of about 40 μM.

Crystallization of Blc was achieved at pH 7.5 with PEG 10000 as precipitant. The obtained crystals belonged to the space group I4₁22 containing one molecule per asymmetric unit. These crystals showed a lattice packing different from the previously described crystals of Blc-X in space group P2₁2₁2₁ with two molecules in the asymmetric unit. Interpretable main chain electron density for Blc was observed for 149 of 168 residues present in the construct. Missing residues comprised the N-terminal amino acids 1 to 8 and the C-terminal amino acids 158 to 168, i.e. the entire Strep-tag

The overall structure of the recombinant Blc analyzed here was very similar to the one of Blc-X (PDB entries 1QWD and 2ACO), revealing the typical lipocalin fold characterized by a β-barrel with eight anti-parallel strands (designated A-H) and a C-terminal α-helix (FIG. 1). In contrast, however, our structure clearly lacked the first two artificial β-stands (designated -1 and 1, respectively) outside the β-barrel that were previously described for Blc-X. This Blc variant, Blc-X, carried 18 additional residues at the N-terminus (thereof 9 visible in the crystallographic model; PDB entry 2ACO) as well as 4 amino acid replacements at the beginning of the mature sequence, both originating from the attBl Gateway® recombination sequence on the expression vector pDest17 (Campanacci et al., 2006). Together with a stretch of the following native sequence, a two-stranded extra antiparallel β-sheet is formed in the crystal structure of Blc-X (FIGS. 1 and 2). The recombinant Blc prepared in the present study, however, lacked this extra peptide segment and had an almost native N-terminus, except for the missing lipid anchor at Cys 1 (data not shown).

Superposition of Blc with the two non-symmetrical monomers A and B of the Blc-X crystal structure (PDB entry 2ACO) resulted in an r.m.s.d. (over 149 Cα positions) of 0.68 Å and 0.89 Å, respectively, while mutual superposition of the latter two monomers yielded an r.m.s.d. of 1.07 Å. Beside the artificial N-terminus in Blc-X, the largest conformational differences were observed in the E/F-loop, which was well ordered in our structure. This loop, which connects strands E and F of the β-barrel at its open end, adopted a distinct conformation in each of the three X-ray structures, apparently influenced by the differing crystal-packing environment (FIG. 1). Further deviations were observed at the loop regions connecting β-strands A-B and C-D, indicating increased flexibility, which has also been described for other members of this protein family, for example human tear lipocalin (Breustedt et al., 2009). The differing conformations of the A/B- and E/F-loops are critical for ligand binding as their arrangement restricts the accessibility of the deep ligand pocket. Only in monomer B of the Blc-X crystal structure the A/B- and E/F-loops adopt a conformation that allows ligand binding. In our Blc structure and in monomer A of the Blc-X structure, the cavity is mostly shielded by the three Phe residues 35, 90, and 91.

Example 2 Reassessment of the Proposed Dimerization Mechanism for Blc

Previous studies of Blc-X indicated a dimeric state both in the crystal lattice as well as in solution, suggesting formation of an asymmetric functional homodimer with different affinities of its two subunits for lipid ligands (Campanacci et al., 2004; Campanacci et al., 2006). In contrast, we observed Blc as a monomeric protein not only in the new crystal form, but also in solution. To thoroughly compare the previously published Blc-X dimer interface with similar crystal packing contacts in our Blc structure, we performed molecular surface and interaction analyses using PISA (Krissinel & Henrick, 2007).

The dimer interface of Blc-X (PDB entry 2ACO) is formed by 19 and 22 residues for molecules A and B, respectively, involving the N-terminal extra peptide segment together with the ET loop and leading to 709 Å² and 792 Å² buried surface area (BSA). These numbers are smaller than the ones of 786 Å² and 825 Å² reported before (Campanacci et al., 2006), which may be attributed to different, in part undisclosed, algorithms used. However, an even larger discrepancy was observed for the total solvent accessible surface area (ASA) of the two Blc-X monomers. Using PISA, we calculated a total ASA of 8635 and 8784 Å², for molecules A and B, respectively, compared with the published number of 7800 Å² per monomer (Campanacci et al., 2006). Similar larger values were obtained with the programs DSSP (Kabsch & Sander, 1983) and ArealMol (CCP4, 1994).

Based on our calculations, on average 8.6% of the total ASA of Blc-X becomes buried at the interface of the two monomers A and B. In contrast, in the new I4₁22 crystals Blc forms its tightest contact with a symmetry-related monomer in a neighbouring unit via a different surface region, around the N-terminus of strand A and the preceding loop that crosses the bottom of the β-barrel (not shown). This contact is accompanied by a significantly smaller BSA of 569 Å², which corresponds to merely 7.0% of the total ASA (8077 Å²).

A striking feature of the previously described Blc-X dimer interface is the two-stranded antiparallel β-sheet that originates from the artificial 22 N-terminal residues. This small extra β-sheet itself neither interacted with the second monomer nor was involved in interactions with symmetry-related neighbours.

However, the hairpin loop that connects the two β-strands seemed to be important for dimer formation of Blc-X (FIG. 2): the loop of molecule B intimately interacted with molecule A, while the same loop of molecule A did not participate in an equivalent interaction, in line with the asymmetry of the dimer noted before (Campanacci et al., 2006). When all the artificially introduced N-terminal residues of molecule B were omitted from the surface analysis, the BSA became significantly reduced to 608 and 686 Å² for molecules A and B, respectively, that is 7.0% and 8.0% of the total ASA. These smaller values would hardly be significant for a true oligomeric state (Miller et al., 1987). Moreover, the latter numbers were in a similar range as another crystal contact in the P2₁2₁2₁ space group with BSAs of 618 Å² and 606 Å² for molecules A and B, respectively, corresponding to 7.2% and 7.0% of ASA.

Taken together, two structural features became evident for Blc-X: (i) its N-terminal extension gives rise to the additional β-sheet, which conformationally fixes the loop in between, and (ii) substitution of the first four residues Cys-Ser-Ser-Pro of wild-type Blc by Lys-Ala-Gly-Ser within this loop leads to a unique intermolecular interaction (FIG. 2) which possibly also stabilizes Blc-X dimer formation in solution. In contrast, the C-terminal Strep-tag II, employed for affinity purification of Blc in our study, is far away from the N-terminus (at least 35 Å distance) and structurally disordered, thus clearly lacking defined interactions with neighbouring molecules in the I4₁22 crystal lattice.

The dimer interface of Blc-X was further dominated by the interaction between the E/F-loops of monomers A and B, which contributed 375 and 422 Å² BSA, respectively, corresponding to ca. 53% of the total contact region including the N-terminal hairpin loop. Due to the asymmetry of the dimer the E/F loop adopted a distinct conformation in each monomer (FIG. 1). Analysis of the alternative crystal packing of Blc in the space group I4₁22 revealed that a comparable but distinct contact occurred there with a symmetry mate, related via a crystallographic two-fold axis (data not shown). This crystal contact showed a total BSA of 480 Å² and was again dominated by the E/F-loop (residues 88-96), with a local BSA of 336 Å² on each molecule corresponding to 70% of the total BSA in this region. Notably, in the new Blc structure the E/F-loop showed a conformation different from both Blc-X monomers (FIG. 1), suggesting structural flexibility. Therefore, dimerization via the E/F-loop should be entropically disfavoured in solution. The fact that a crystal contact involving the E/F-loop was observed in both crystal structures may be solely attributed to its largely hydrophobic nature.

Example 3 Comparison of Blc to ApoD

Mammalian apolipoprotein D (ApoD) is the closest eukaryotic homologue of Blc and also anchored in a lipid micelle, albeit via a different mechanism (Eichinger et al., 2007).

Structural comparison between Blc and ApoD resulted in 139 matching Cα-positions out of 149 resolved residues in the Blc structure, with an overall r.m.s.d. of 1.37 Å (data not shown). Differences between the two lipocalin structures were mainly observed in the strand connecting loop regions at the open end of the β-barrel. The loops A/B and, in particular, G/H showed the largest deviations. Loop G/H formed an extended hairpin structure in ApoD resulting in a wider pocket, ideally shaped for accommodation of a steroid ligand. On the other hand, the A/B loop, which has a one-residue insertion in Blc compared to ApoD, partially shielded the ligand pocket in the bacterial counterpart. Beside variations in pocket size and accessibility, there were differences in surface hydrophobicity. While ApoD showed distinct hydrophobic patches, which were likely involved in high density lipoprotein (HDL) micelle association (Eichinger et al., 2007), hydrophobic surface areas of Blc were mainly confined to the interior of its cavity. This is in agreement with Blc's presumed function to bind fatty acid-like ligands (Campanacci et al., 2006) whereas membrane association by its N-terminal lipid anchor.

Example 4 Blc Vector Construction

The coding sequence for Blc was amplified from genomic DNA of E. coli K12 strain TG1/F⁻ (Kim et al., 2009) via PCR according to a published procedure (Skerra, 1992) by using phosphorothioate primers 5′-CCG CCA GTT CTC CTA CGC CGC CG-3′ (also introducing the Cys1 to Ala mutation) (SEQ ID NO. 5) and 5′-GCT ACC AGG CTG CTG TAC CC-3′ (SEQ ID NO. 6) The unique amplification product was purified by agarose gel electrophoresis, phosphorylated with T4 polynucleotide kinase (New England Biolabs, Beverly, Mass.), and ligated with the expression vector pASK75-strepII (Skerra, 1994; Schmidt & Skerra, 2007), which had been cut with StuI and Eco47III and dephosphorylated using shrimp alkaline phosphatase (USB, Cleveland, Ohio). After transformation of E. coli XL1-Blue (Bullock et al., 1987) the resulting plasmid, designated pBlc1, was isolated and its composition was confirmed by restriction digest as well as double-stranded dideoxy-sequencing (ABI PRISM 310 Genetic Analyzer; Applied Biosystems, Foster City, Calif.). On pBlc1, the recombinant protein was encoded in fusion with the amino-terminal type I signal peptide of OmpA and the C-terminal Strep-tag II of nine residues (Breustedt et al., 2006; Schmidt & Skerra, 2007). The codon for the unpaired internal thiol residue Cys113 (numbering according to the mature full length protein; Swiss-Prot entry P0A901) was subsequently replaced by a Ser codon via site-directed mutagenesis (Geisselsoder et al., 1987) with the oligodeoxynucleotide 5′-GGT CCG GCC CGC TAA CCA GCG CAT G-3′ (SEQ ID NO. 7), finally yielding pBlc2, which was used for recombinant protein production throughout this study.

Example 5 Blc Protein Production and Purification

Recombinant Blc was produced in the E. coli K-12 strain JM83 (Yanisch-Perron et al., 1985) harbouring pBlc2 by secretion as a soluble protein into the bacterial periplasm. Shake flask cultures were grown in 21 LB medium supplemented with 100 mg/l ampicillin at 22° C. Gene expression was induced at a cell density of OD₅₅₀=0.5 by adding 0.2 mg/l anhydrotetracycline (Skerra, 1994). After further shaking for 3 h the cells were harvested by centrifugation, resuspended in 500 mM sucrose, 1 mM EDTA, 100 mM Tris-HCl pH 8.0, and kept on ice for 30 min. The resulting spheroplasts were sedimented by centrifugation and the supernatant containing the recombinant protein was recovered. The protein extract was dialyzed against 150 mM NaCl, 1 mM EDTA, 100 mM Tris-HCl pH 8.0 and applied to a Strep-Tactin affinity column (Schmidt & Skerra, 2007) using the same buffer. The recombinant Blc was competitively eluted by application of 2.5 mM D-desthiobiotin in the chromatography buffer. Elution fractions were concentrated, applied to a preparative Superdex 75 gel filtration column (GE Healthcare, Uppsala, Sweden) using 150 mM NaCl, 1 mM EDTA, 100 mM Tris-HCl pH 8.0 as running buffer, and eluted in a homogeneous peak. The yield was ca. 1.5 mg of purified protein per 11 E. coli culture.

Example 6 Blc Biochemical Characterization

Analytical size exclusion chromatography (SEC) was carried out on a Tricorn S75 column (Superdex 75 10/300 GL, bed volume V_(t)=24 ml; GE Healthcare) at a flow rate of 0.5 ml/min using ÄKTA Purifier instrumentation (GE Healthcare) with PBS (4 mM KH₂PO₄, 16 mM Na₂HPO₄, 115 mM NaCl) as running buffer. Bovine serum albumin (66 kDa, V_(r)=9.861 ml), carbonic anhydrase (29 kDa, V_(r)=12.35 ml), myoglobin (17.05 kDa, V_(r)=13.244 ml), cytochrome C (12.4 kDa, V_(r)=14.11 ml), and aprotinin (6.5 kDa, V_(r)=16.35 ml) were used as protein size standards for calibration of the column while the void volume was determined with blue dextran (V₀=8.08 ml).

Sedimentation equilibrium experiments were performed using an XL-I analytical ultracentrifuge and a Ti-60 rotor equipped with a UV/Vis as well as interference detector (Beckman, Fullerton, Calif.). An 0.8 mg/ml solution of the purified recombinant Blc in 150 mM NaCl, 1 mM EDTA, 100 M Tris/HCl pH 8.0 was applied to six-sector 12 mm path length cells. The samples were centrifuged at 25000 rpm for 72 h at 4° C., until equilibrium was reached, whereby the protein gradient was measured by UV absorption at 280 nm. Data analysis was carried out with Kaleidagraph software (Synergy Software, Reading, Pa.) as previously described (Zander et al., 2007; Stromer et al., 2004) using a value of 0.73 ml/g for the specific volume of the protein.

Example 7 Blc Crystallization and Structure Determination

Blc crystals were grown in hanging drops using the vapour diffusion technique. Drops mixed from 1 μl protein solution (10 mg/ml, dialyzed against 10 mM Tris-HCl pH 8.0) and 1 μl reservoir solution were equilibrated against 0.5 ml reservoir solution on siliconized glass cover slips. After about two months at 20° C., two crystals were obtained in the presence of 20% (w/v) PEG 10000, 100 mM HEPES-NaOH pH 7.5. Blc crystals were harvested using Nylon loops (Hampton Research, Laguna Niguel, Calif.), cryo-protected with Paratone N (Hampton Research)—thereby removing excess mother liquor—and frozen in a 100 K nitrogen stream (Oxford Cryosystems, Oxford, UK).

A native data set was collected on a mar345 imaging plate detector (MarResearch, Hamburg, Germany) using monochromatic Cu-K_(α) radiation from a RU-300 rotating anode generator (Rigaku, Tokyo, Japan) equipped with Confocal Max-Flux Optics (Osmic, Troy, Mich.). Diffraction data were processed with the XDS Package (Kabsch, 1993). The Blc crystals belonged to the space group I4_(t)22 with unit cell parameters a=b=88.94 Å, and c=78.35 Å, containing one protein molecule per asymmetric unit (Table 1). The X-ray structure was solved by molecular replacement as implemented in PHASER (Storoni et al., 2004) using the coordinates of a published Blc structure (PDB code 1QWD) after deleting the N-terminal residues −17 to 4 as well as loop residues 33 to 38 and 60 to 69 at the open end of the β-barrel. Model building was performed with Coot (Emsley & Cowtan, 2004), followed by restrained and TLS refinement using REFMAC5.5 (Murshudov et al., 1997; Winn et al., 2001). Total B values were calculated with TLSANL (Howlin et al., 1993). Finally, the structure was validated with Coot and MolProbity (Davis et al., 2007).

Graphics were prepared with PyMOL (DeLano, 2002) while secondary structure elements were assigned with DSSP (Kabsch & Sander, 1983). Superposition of structures was performed with SUPERPOSE (Krissinel & Henrick, 2004) and interfaces were analyzed with PISA (Krissinel & Henrick, 2007). The coordinates and structure factors for the refined Blc structure have been deposited at the RCSB Protein Data Bank (PDB accession code 3MBT).

REFERENCES

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TABLE 1 Data collection and refinement statistics for Blc. Data collection Space group l4₁22 Unit cell parameters [Å] a = b = 88.94, c = 78.35 Wavelength [Å]  1.5418 Resolution [Å] 30-2.6 (2.7-2.6)^(a) Completeness [%] 98.7 (99.8) Unique reflections 5024 (531) Multiplicity 6.1 (6.2) Mean l/σ(l) 34.7 (9.4) R_(meas) [%]^(b) 4.5 (20.1) Wilson B-factor [Å²] 36.4 Refinement Resolution [Å] 18.70-2.60 (2.67-2.60)^(a) Reflections (working) 4787 (343) Reflections (test) 236 (16) R_(cryst) [%]^(c) 22.2 (30.6) R_(free) [%]^(d) 27.6 (46.0) Number of protein atoms/water molecules 1209/18 B-values of protein atoms/water mols [Å²] 36.0/11.7 Ramachandran plot: favoured/outliers [%] 93.2/0.0 Rmsd bonds [Å]/angles [°] 0.009/1.227 ^(a)Values in parentheses represent the highest resolution shell. ^(b) $R_{meas} = \frac{\sum\limits_{hkl}\;\sqrt{\frac{n}{n - 1}{\sum\limits_{j = 1}^{n}\;{{l_{{hkl},j} - \left\langle l_{hkl} \right\rangle}}}}}{\sum\limits_{hkl}\;{\sum\limits_{j}\; l_{{hkl},j}}}$ ^(c) $R_{cryst} = \frac{\sum\limits_{hkl}\;{{F_{hkl}^{obs} - F_{hkl}^{calc}}}}{\sum\limits_{hkl}\; F_{hkl}^{obs}}$ ^(d)R_(free) is calculated as for R_(cryst), but with 5% of the reflections excluded from the refinement 

What is claimed is:
 1. A composition comprising a plurality of monomeric bacterial lipocalin (Blc) muteins, wherein said Blc muteins differ from each other in at least one amino acid at a position within at least one loop region of a Blc polypeptide, wherein the four loop region of said Blc polypeptide are at amino acid positions corresponding to positions 27-43, 58-73, 88-96, and 113-121 of the mature Blc polypeptide set forth in SEQ ID NO: 4, wherein said plurality of Blc muteins comprise at least 50 different polypeptide molecules, and wherein said Blc muteins are within at least 75% sequence identity with SEQ ID NO:
 4. 2. The composition according to claim 1, wherein said Blc muteins differ from each other in at least one amino acid at a position within at least two loop regions of said Blc polypeptide.
 3. A method of isolating a polypeptide of interest, comprising the steps of (i) allowing the composition of claim 1 to be in contact with a target of interest, wherein said polypeptide of interest is comprised within said composition and specifically binds to the target; and (ii) isolating the polypeptide or a polypeptide-target complex resulting from said specific binding. 